A radio access connection in a cellular communication system typically stretches between a base station and a user terminal served by the base station. A relay node is intended to give increased coverage (i.e., to extend the radio access connection) without the need to install a further base station.
When a relay node is included in a radio access connection between a base station and a user terminal, the base station is referred to as a donor base station. The communication link stretching between the donor base station and the relay node is referred to as backhaul link (also denoted Un), whereas the communication link stretching between the relay node and the user terminal is referred to as the access link (also denoted Uu).
A relay node has two transmitters and two receivers, one each for the backhaul link and the access link. In general, care has to be taken when operating the two links in order to ensure that they do not interfere with one another. In particular, it has to be ensured that the receiver for one link does not experience strong interference from the transmitter for the other link (“self-interference”).
FIG. 1 illustrates an exemplary communication scenario involving a base station 10, a user terminal 20 and a relay node 30 extending a radio access connection 40 between the base station 10 and the user terminal 20. The relay node 30 defines a relay cell 50 in which the user terminal 20 is served. In the downlink (DL) from the base station 10 to the user terminal 20, the relay node 30 receives data from the base station 10, decodes and re-encodes it, and then transmits the re-encoded data to the user terminal 20. In the uplink (UL) the same steps are performed by the relay node 30, but in the other direction from the user terminal 20 to the base station 10. The relay node 30 can generally have a similar output power as the base station 10. Still, there are many deployment scenarios in which a significantly lower output power of the relay node 30 will suffice.
One alternative to a relay node is a repeater. In a repeater, the data is not decoded and re-encoded, but rather the signal is just amplified and re-transmitted. For this reason repeater operation is also referred to as Amplify and Forward (AF), whereas relay operation is commonly referred to as Decode and Forward (DF).
A repeater will be faced with the problem that it needs to receive a rather weak signal at the same time as it is transmitting a signal that is considerably stronger. While the power of the received signal might be in the order of −80 dBm, the power of the transmitted signal might be in the order of 0 dBm. To avoid self-interference and a resulting self-oscillation, this puts rather hard requirements on the amount of isolation required. As a rule-of-thumb, the isolation should be about 10 dB higher than the amplification of the signal. For instance, if the amplification is 80 dB as in the example above, then the isolation should be about 90 dB.
In Eun-Ji Yoo et al., “Self-Interference Cancellation Method based on V-BLAST in MIMO Systems”, ICACT 2009, p. 800-803, Feb. 15-18, 2009 (ISBN 978-89-5519-139-4) a technique for cancelling self-interference in a repeater is described. The technique comprises the establishment of a feedback channel between an input and an output of an amplifier of the repeater. In addition, an adaptive filter is provided that is based on a Least-Mean-Square (LMS) algorithm for cancelling self-interference.
It has been found that there is a crucial difference between a repeater and a relay node when it comes to the fundamental mechanisms underlying self-interference. Since a repeater does not decode the information, the requirements for self-interference suppression will be set by the quality of the transmitted (amplified) signal. For a relay node, on the other hand, the received signal needs to be decoded, which means that the requirements will be determined by the requirements of the receiver in the relay node.
There are two different approaches for a relay node to avoid or at least significantly reduce interference, and in particular self-interference. According to a first approach, the two links use different frequency bands (out-of-band relaying), in which case coexistence of the two links is ensured by means of filtering. According to a second approach, the two links use different time slots in a frame structure so that coexistence is ensured by means of scheduling (in-band relaying).
The second approach, which is based on the use of different time slots, implies restrictions on the maximum data rate as well as on the scheduling of the individual transmissions. One drawback of the first approach is the fact that different frequency bands are needed for the two links. This need implies that twice the amount of spectrum is required for operating both links (compared to a communication scenario without a relay node). Also, the frequency separation between the two links must be rather large to make filtering feasible. If the separation is not large enough, so that no filtering can be applied in the receivers of the relay node, the relay node will experience strong self-interference.